Purification of latex



Patented Apr. 16, 1946 PURIFICATION OF LATEX Hendrik Roelofi Braak,Batavia-Centrum, Java,

Dutch East Indies, assignor, by mesne assignments, to P. Honig,Washington, D. 0., Commissioner of the Board for Economic and FinancialAfiairs of The Netherlands Indies, Surinam and Curacoa, as trustee NoDrawing. Application November 16, 1940, Se-

rial No.- 366,009. In the Netherlands October This invention relates topurification of latex; and it comprises a method of purifying latices ofrubber, gutta percha, balata, jelutong, abiurana, gondang and the like,said method comprising subjecting such a latex to a treatment withalkali for purification purposes, separating the impurities of highspecific gravity precipitated by the alkali, this separation beingusually accomplished by the use of a centrifuge or by settling, wherebymetallic compounds and other impurities are substantially eliminated,this resulting in the production of a latex of enhancedpurity, greaterstability and of more constant composition; said separating step beingadvantageously conducted While the latex is heated, being sometimesconducted in an inert atmosphere and being followed usually by dialysisof the latex to further increase its purity; all as more fullyhereinafter set forth and as claimed.

A Well known method for the purification of latex consists in treatingthe latex with alkali, preferably at elevated temperatures eventuallyfollowed by other operations. My prior copending application, Serial No.281,264, filed June 26, 1939, describes such a treatment, comprising theheating of latex under normal pressure with a caustic lye at boilingtemperature, followed by dialysis of the alkali-treated latex. By thelatter process two distinct advantages are obtained; viz, not only is alatex obtained which is much purer than the original latex, but, inaddition, it has been found that the composition (nitrogencontent,ash-content, etc.) and properties of this purified latex, as well as therubber obtained therefrom by coagulation (e. g. its resistance to waterabsorption) are far less dependent upon that the Hevea tree reacts tothese variations in a physiological manner, yielding during thoseperiods a latex of abnormal composition (particularly a high nitrogenand ash-content) This is particularly true for the period of leaf-fall(see also Norman Rae in The Analyst, 53, p. 330: Seasonalvariations inthe composition of the latex of Hevea brasz'liensz's).. Under these ex-9 Claims. (Cl. 260-816) ceptional circumstances, certain variationsoccur even in the latex obtained according to my acknowledgedapplication, Serial Number 281,264. These variations, although small incomparison with the variations in the original latex, are beyond thevariations which are to be considered as normal for this purified latex.For instance, it was observed that the nitrogen-content in the crepeprepared from such a purified latex may under those circumstances riseto 0.14 per cent-0.15 per centinstead of the "usual 0.08-0.10 per cent.Similarly the ash-content'may rise to 0.16 per cent instead of the usual0.10 per cent. While investigating this matter, it was found that thetreatment of the latex with alkali, especially at elevated temperaturesapproaching its boiling point, causes a certain portion of the nonrubberconstituents, inorganic aswell as organic (including nitrogen-containingcompounds) to be precipitated in an insoluble or less soluble form; itwas also found that the higher the original content of non-rubberconstituents in the latex, thelarger in general the precipitate. Thelatter is, e. g. the case in the period of, or preceding, the leaf fallof the Hevea.

Further investigation showed that the quality of such latex, after suchtreatment with alkali,

can be improved by the elimination of said precipitated substances by amethod based on their differences in specific gravity as compared withthe liquid, This elimination can be accomplished by mere settling.However, it has proved preferable to perform the elimination in aclarifying centrifuge and at temperatures approaching its boiling point,since by, the latter method additional substances can be eliminatedwhich by their specific gravity or fineness settle very incompletely orslowly. Any type of suitable centrifuge built for the separation ofsolid particles from a liquid, may be used, provided the construction issuch that. no difficulties, caused by the coagulation of rubber from thelatex, occur. The absorption of air by the latex during the operationshould be avoided as far as possible. If desired the separation may becarried out. in a closed centrifuge in an atmosphere of nitrogen orother inert gas, although in general this precaution is not necessary.

Further it was found that the solid matter separated in the centrifugeoften. contains metallic compounds known to unfavorably influence theaging and other properties of rubber, such as copper, manganese, iron,etc. Apparently the metallic compounds in the original latex areconverted during the alkali-treatment either partially or Althoughseveral methods of treating latex with alkali have been described in thecurrent literature, none of these suggests the elimination of theinsoluble compounds which are formed during the alkali treatment.

The latex obtained by the process according to the present invention isa very suitable starting material for various applications. For example,this latex may be submitted advantageously to the process of dialyzingas described in my following claims will be immediately evident tothoseskilled in this art.

What I claim is:

1. In the purification of latices of rubber, balata,'gutta percha,jelutong, abiurana, gondang and the like, the process which comprisestreating such a latex with a dilute caustic alkali at elevatedtemperatures approaching its boiling point to cause a precipitation ofheavy impurities therein, separating a small fraction containing saidheavy impurities without substantial change in the rubber content of thelatex, and then dialyzing the so-purified latex to further purify thesame.

2. The process of claim 1 wherein the dialyzing treatment is attemperatures approaching the boiling point of the latex.

acknowledged application. It has beenfound that, by centrifuging thelatex after treatment with lye and then dialyzing, it is possible toobtain a normal latex even in the abnormal periods of seasonalvariations. It has also been found that latex tapped in normal seasonsis improved, especially with respect to its nitrogen and ash contents inthe crepe obtained by coagulation, and a better resistance against waterabsorption in the latter.

In a specific embodiment of this invention, which represented apractical operation, I first passed a fresh latex through a screen toremove dirt and the larger impurities. This latex was then mixed with asolution of sodium hydroxide, having a concentration of 33 per cent byweight, in the proportions of 30 parts by weight of solution to 1000parts by weight of the latex. Steam was then passed into the mixture tohold it at boiling temperature for a period of 2 hours. The so-treatedlatex was then passed directly through a centrifuge, while still at anelevated temperature, and a small fraction of heavy material wasremoved, this fraction containing the impurities precipitated in thecaustic treatment. The centrifuged latex was then diluted to a solidscontent of 20 per cent and the diluted latex was heated again to itsboiling point and passed through seamless Cellophane tubing in adialyzing apparatus in counterflow to a stream of water maintained atsubstantially its boiling point. The sopurified latex was then dilutedto a concentration of about 5 per cent and subsequently coagulated witha weak acid, the coagulum being worked up into crepe rubber.

It has been found that removal of the heavy impurities precipitated inthe caustic treatment results in the elimination of many of thedifiiculties which are normally encountered in the dialyzing step ofpurifying latex. It has been found that these difficulties are caused bythe clogging of the dialyzing tubes, not only by the settling andaccumulation of the heavy impurities in the tubes but also owing .to thefact that these impurities promote local coagulation of the latex,resulting in the plugging of the tubes. The present invention avoidsthese difiiculties by removal of the heavy impurities prior to thedialyzing step.

While I have described what I believe to be the best embodimentsof myprocess, it is obvious that various modificationscan be made in thespecific procedures described Without departing from the purview of thisinvention. Many such modifications which fall within the scope of the 3.The process of claim 1 wherein said small fraction containing heavyimpurities is separated at elevated temperatures approaching the boilingpoint of the latex.

4. The process of claim 1 wherein the treatment with alkali and thedialyzing treatment are conducted at elevated temperatures approachingthe boiling point of the latex.

5. The process of claim 1 wherein the alkali treatment, the separationof heavy impurities and the dialyzing treatment are conducted atelevated temperatures approaching the boiling point of the latex.

6. The, process of claim 1 wherein the separation of heavy impurities isconducted in the presence of an inert atmosphere.

7. In the purification of latices of rubber, balata, gutta percha,jelutong, abiurana, gondang and the like, the process which comprisesboiling such a latex in contact with a dilute caustic alkali for a timesufiicient to cause the precipitation of heavy impurities, centrifugingsaid latex to remove a small fraction containing said heavy impuritieswithout substantial change in the rubber content of the latex, thendialyzing the latex at elevated temperatures approaching its boilingpoint to further purify the same.

8. In the purification of latices of rubber, balata, gutta percha,jelutong, abiurana, gondang and the like, the process which comprisesboiling such a latex in contact with a dilute caustic alkali for a timesuificient to produce the precipitation of heavy impurities,centrifuging said latex at temperatures approaching its boiling point toseparate a small fraction containing said heavy impurities withoutsubstantial change in the rubber content of the latex, then dialyzingthe latex at elevated temperatures approaching its boiling point.

9. In the purification of' rubber latex, the process which comprisesmixing a rubber latex with a caustic sodasolution in proportions toprovide about 30 parts by weight of a solution containing about 33 percent of caustic soda to about 1000 parts by weight of the latex, boilingthe resulting mixture for about 2 hours, passing the resulting productdirectly to a centrifuge and separating a small fraction containing theheavy impurities precipitated by the caustic treatment, diluting the sotreated latex to a solids content of approximately 20 per cent, heatingthe latex approximately to its boiling point and dialyzing it in tubesof Cellophane in counterflow to a stream of water maintainedsubstantially at its boiling point to further purify the latex.

HENDRIK ROELOFF BRAAK.

